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Fishery Bulletin 93(2), 1995 



fishing trip in 1964 were recorded and reported by a 

 fisherman who may have reported his observation 

 because of the magnitude of the kill, biasing those 

 data (Smith and Lo, 1983). However, mean MPS's 

 from these three trips were mostly within the range 

 of values from the other observed trips in 1971-72 

 and therefore do not appear biased (Table 2). 



Removing the data from the three trips observed 

 prior to 1971 resulted in minimal changes to the 

 mortality estimates. Annual mortality was higher in 

 each year except 1972. This resulted in an estimate 

 of total dolphin kill for 1959-72 of 5.1 million versus 

 an estimate of 4.9 million when those trips were in- 

 cluded. This confirms that my kill estimates were 

 not significantly biased by the three pre- 1971 ob- 

 served trips. 



Accepting the assumption of constant MPS rates 

 during 1959-72 is crucial to the accuracy of these 

 kill estimates. Available evidence is consistent with 

 this assumption. For example, the three observed 

 trips from 1964, 1965, and 1968 had MPS rates in 

 the same range as the data collected in 1971-72. 

 There are additional observations prior to 1971 that 

 are also consistent with this assumption. For ex- 

 ample, dolphins were noted as being killed in all 28 

 sets observed on a trip in 1966, but precise counts 

 were made for only the five sets on which the great- 

 est number of dolphins were killed (Smith and Lo, 

 1983). The average MPS for those five sets was 250.0; 

 thus a minimum estimate of the average MPS for 

 that trip is 45.5, if one assumes that only one dol- 

 phin was killed during each of the other 23 sets. 

 Perrin (1969) reported a "rough count" of the total 

 number killed on the trip as 2,000, resulting in an 

 average MPS of 71.4. Most of those sets were known 

 to be successful sets using backdown procedure, and 

 values of 45.5 or 71.4 are within the range (44.2 to 

 142.5) reported for the small vessel stratum on the 

 1964-68 trips (Table 2). During the same period of 

 1964-68, another scientist reported observing two 

 trips with similar MPS rates (Allen 9 ). Therefore, 

 there are apparently at least four other observed fish- 

 ing trips from the 1960's with kill rates similar to 

 those reported here. Finally, fishermen on the 1965 

 trip and the 1966 and 1968 trips reported to observ- 

 ing scientists that the MPS rates on those trips were 

 the usual rates experienced by the crew of those ves- 

 sels and others in the fleet at that time 1011 . 



Perhaps the greatest uncertainty is due to the lack 

 of MPS data prior to 1964. Historically, MPS rates 



9 Allen. R., pers. commun., cited in Smith and Lo, 1983. 



10 1965 trip, David W. Waller, Dept. of Biol. Sci., Kent State Univ., 

 Kent, Ohio. Pers. commun., 1994. 



11 1966 and 1968 trips, William F. Perrin, Southwest Fish. Sci. 

 Cent., La Jolla, CA. Pers. commun., 1994. 



have decreased in the U.S. fleet because of improve- 

 ments in the equipment and in the skill and motiva- 

 tion of the fishermen. Use of the backdown proce- 

 dure was developed on one boat in 1959—60, and its 

 use spread quickly through the majority of the fleet 

 only after 1961 (Barham et al., 1977). Because the 

 procedure saved them time in retrieving the net 

 (Barham et al., 1977), the fishermen likely improved 

 their backdown performance during these first few 

 years. Therefore, the average MPS during backdown 

 sets prior to 1964 may have been higher than in later 

 years. This is consistent with the observation that 

 the proportion of dolphins killed in the net was high 

 during these early years relative to later years (Jo- 

 seph and Greenough, 1979). For this reason, any 

 substantial bias in these kill estimates is likely a 

 negative bias; the actual kill of dolphins may have 

 been higher, particularly prior to 1964. 



Passage of the Marine Mammal Protection Act in 

 1972 motivated fishermen to kill fewer dolphins, and 

 I expected MPS to decline in 1973, which it did. My 

 calculations of MPS in 1973 for R. 2lv ^. 22 .- ^. m . 

 andi?. 12 . were 13.1, 7.9, 22.0, and 1.3, respectively, 

 compared with analogous values of 65.8, 5.9, 29.9, 

 and 0.4 in 1959-72 (Table 2). The only dramatically 

 different value was that of the MPS for small ves- 

 sels with successful set and with backdown proce- 

 dure, which declined from a value of 65.8 to 13.1. 

 The other values are fairly similar. Kill rates on small 

 vessels prior to 1973 were apparently more than 

 twice as high as those on large vessels. Therefore, it 

 seems reasonable that the most dramatic improve- 

 ment in MPS rates would have occurred in this cat- 

 egory. One technological reason for a decline in kill 

 rates at that time was the increasing use of the 

 Medina panel, an area of finer mesh net in the 

 backdown channel that helped prevent entanglement 

 of dolphins. It was first used experimentally in 1971; 

 by the end of 1972, 40-50% of the U.S. fleet were 

 using it, and 60-70% were using it by the end of 1973 

 (Barham et al., 1977). In summary, although data 

 available from 1959-72 are sparse, they are consis- 

 tent with other information available from that pe- 

 riod and 1973, which provides support for accepting 

 the assumptions of this analysis and, therefore, accept- 

 ing these estimates as being reasonably accurate. 



Except for the years 1971 and 1972, the coefficients 

 of variation (CV) for the total number of dolphins 

 killed in each year were considerably higher than 

 those reported by Lo and Smith ( 1986). Since the data 

 in each case are the same, the differences must be 

 due to the use of the bootstrap method versus the 

 analytical equations used in Lo and Smith (1986). 

 The differences in CVs were largest for years 1959- 

 65. For example, they report a CV of 0.31 for 1960 



